1. Field of the Invention
The present invention relates to a projection exposure apparatus such as the photo repeater used for making working masks for use in the manufacture of semiconductor elements and the wafer stepper used for the exposure of semiconductor wafers. More specifically, the present invention is directed to a reducing projection type exposure apparatus particularly useful as the wafer stepper.
2. Related Background Art
In these years, many reducing projection type exposure apparatus, called a stepper, have been used in the manufacturing line of semiconductor elements.
The stepper is an optical apparatus for exposing a light image of a circuit pattern described is a reticle or a photomask on a wafer through a reducing projection lens at a reduction of 1/5 or 1/10.
In the stepper, the size of the area on which a pattern is copied by one shot of exposure is determined by the size of the image field of the projection lens. Usually the exposure area size is smaller than the whole surface area of a wafer. Therefore, the wafer stage having a wafer thereon is moved stepwise after every exposure so that the next exposure may be made on the next area of the same wafer. This operation is repeated for the same wafer a number of times. This projection exposure method is known as the step and repeat method.
As one of the methods for the alignment of wafer with a reticle, there has been known and used the so-called off axis alignment method. According to the method, a wafer alignment microscope is provided at a position a certain distance spaced from the optical axis of the projection lens. Using the wafer alignment microscope, one detects an alignment mark on the wafer. After the detection, one moves the wafer stage, which can move two-dimensionally, to the position in which an arbitrary exposure area on the wafer gets in alignment with the pattern image of the reticle formed by the projection lens. This positioning of the wafer stage is carried out, using the position of the wafer in which the above alignment mark has been detected, as the base position. After positioning the wafer stage, one carries out an exposure.
The thing essential for high accurate positioning of the wafer stage is precisely detect the distance conventionally called the "base line measure value". The term "base line measure value" means the distance between the reference point projection position (the position at which the reference point on the reticle is projected) and the mark detection center position (the position of the optical axis of the wafer alignment microscope).
To measure the base line measure value, a fiducial mark is provided on the wafer stage. Also, measuring means for detecting the position of the wafer stage, for example, a pair of laser interferometers are provided. The base line measure value is measured using the fiducial mark and the laser interferometers.
I, the inventor of the present invention, have conducted various experiments trying to improve the accuracy of the above measurement. These experiments have led me to the finding that the fluctuation of air around the apparatus is one of the causes for reduction of accuracy in the above measurement of the base line measure value.
The reduction of measurement accuracy has been significant in particular when the space between reticle and projection lens is open and the external air is allowed to free flow in and out across the space. It has been found that in such a construction of projection exposure apparatus, a slight shift of image is caused by the fluctuation of the air in the space and that the magnitude of the image shift is very small but may be sufficient to degrade the measurement accuracy. The image shift caused by the fluctuation of air takes place at random in direction as well as in magnitude. Therefore, it is highly possible that a large image shift is caused just at the time of measuring the base line measure amount. In this case, the image shift will be incorporated into the measured value as an error. As a result, the accuracy of registration of pattern image with wafer exposure area at the time of exposure is decreased to the extent corresponding to the image shift. Of course, such a fluctuation of air occurs also during a real exposure. In this case, the projected image is shifted on the wafer, and even a slight shift may reduce the resolution of fineness of the pattern formed on the wafer.
In order to improve the accuracy of measurement in the projection exposure apparatus, these drawbacks must be overcome at first.